1. Field of the Invention
This invention relates to an image forming unit and an image forming apparatus and, more particularly, to those suitably applicable to such as, e.g., electrophotographic printers (hereinafter, referred to as printers).
2. Description of Related Art
Conventional printers are formed with a driven coupling for drum at the photosensitive drum, and with a driven coupling for roller at the developing roller. In such printers, an output shaft of a motor is coupled with a drive coupling for drum and a drive coupling for roller via plural gears attached to these couplings. The printer is structured to connect the drive coupling for drum with the driven coupling for drum, and to connect the drive coupling for roller with the driven coupling for roller while the surface of the developing roller is pushed to the surface of the photosensitive drum. With this mechanism, the printer transmits the rotation of the output shaft of the motor to the driven couplings for drum and roller via the drive couplings for drum and roller to rotate the photosensitive drum and the developing roller, and electrostatic latent images formed on the surface of the photosensitive drum are developed with toner transferred from the surface of the developing roller to form toner images onto the surface of the photosensitive drum (see, e.g., Japanese Application Publication (A1) No. 2009-116, 153).
With the conventional printer, however, the circumferential speed of the developing roller may be deviated periodically, so that irregular developments may occur on the electrostatic latent images on the surface of the photosensitive drum due to the toner, and there raises a problem that the image quality may be impaired upon occurrences of irregular pitches (hereinafter, referred to as jitters) at the printed images formed from toner images obtained through phenomena of the irregular developments.
In consideration for solving the above problem, it is therefore an object of the invention to provide an image forming unit and an image forming apparatus capable of preventing the quality of printing images from being degraded.